There is known a spark plug for an internal combustion engine, in which a resistor is arranged between a center electrode and a metal terminal in an axial hole of an insulator so as to suppress a radio noise caused by ignition (see, for example, Japanese Laid-Open Patent Publication No. 2003-22886).
In the axial hole of the insulator, a conductive seal layer is disposed between the resistor and the center electrode. For instance, a thermal expansion coefficient of the conductive seal layer is set to a midpoint between a thermal expansion coefficient of the insulator and a thermal expansion coefficient of the center electrode.
In recent years, there is a tendency that load exerted on the spark plug in a usage environment increases with increases in the output and temperature of the internal combustion engine. In such a severe usage environment, it is likely that a malfunction such as crack will occur at an interface of the resistor and the conductive seal layer under the action of thermal stress. This can lead to a deterioration in the durability of the spark plug.
The present description discloses a technique for improving the durability of a spark plug used in an internal combustion engine.